The term SCSI (Small Computer System Interface) may refer to a SCSI-3 set of standards and technologies. SCSI-3 is a suite of standards that define command sets, protocols, and signaling methods such as Fibre Channel, SCSI Parallel Interface (SPI), Serial Attached SCSI, etc. SCSI is not limited to the SCSI Parallel Interface (SPI).
A SCSI I/O comprises a SCSI Command request sent to the SCSI I/O controller along with a SCSI I/O context that is used to associate a response from the SCSI I/O controller with the original SCSI I/O.
With respect to host side queuing with an example of a SCSI HBA (host bus adapter), SCSI I/O contexts are typically stored on the SCSI I/O controller. The host side transport driver typically deals with space for 256 I/O contexts on SCSI I/O controllers and uses a credit based model with the I/O controller in managing the I/O contexts. Whenever the driver runs out of credits on the SCSI I/O controller, the driver queues I/Os (I/O requests and/or commands) in the host memory until some pending I/O requests complete, thus freeing up I/O contexts.